Not long before she disgraced herself, Hazel Blears wrote passionately in a new, pro-Labour blog called LabourList. In a piece entitled "Economic hardship: why the Tories just don't get it", she advertised her party's superior empathy with the poor. "My mother had to take on three part-time jobs to put food on the table," she boasted. "When I look at the Tory benches I think: they just don't get it."

But within months, even Hazel's well-wishers were wondering if Hazel's mother could not have taken on a few more part-time jobs, since three had obviously not been enough for the full poverty experience to sink in. "Hazel Blears seems to have forgotten that many of her constituents survive on a minimum wage of less than £13,000 a year," noted a fellow LabourList blogger, Stephen Newton, after Blears's profound empathy with property speculators was revealed in the Daily Telegraph. "Yet still Hazel simply doesn't get why she has come to symbolise the worst excesses of the MPs' expenses scandal."

That's modern politics for you: one day you get things, the next you just don't. It only takes an accusation in a newspaper column or editorial, a BBC "have your say" special, or a few callers on a phone-in repeating "She just doesn't get it" and a person like Hazel is suddenly lumped in with bankers ("They just don't get it"), moat-cleaning Tories ("They just don't get it"), BBC fat cats ("They just don't get it") and any number of people whose mothers did fewer than three part-time jobs to put food on the table. Denunciations come so fast that you can foresee a time, not far off, when the number of people who just don't get things far exceeds the number of those who do. At current rates of attrition, the only people left who get anything at all, by the end of the year, will be Peter Mandelson, Esther Rantzen and Vince Cable.

For now, perhaps it is some comfort for Hazel in her exile that her fellow eminent non-getters already include Gordon Brown. In fact, if it hadn't already been in use for years, hurled by surly teenagers at their parents, the expression might have been invented for him. Though strict grammatical regulations probably prohibit its appearance on his tombstone, "He Just Didn't Get It" may well prove the lasting epitaph for Brown's irredeemably clumsy style of premiership. After a slowish start, he is now accused of not getting things at least once a week, for reasons that may be anything from smiling in a weird way to ennobling Sugar, head-hunting Glenys Kinnock, worrying about Susan Boyle, not calling an election, staging the Iraq inquiry in secret and, more generally, bringing our country to the brink of moral and economic collapse.

Curiously, its overuse in recent months, in myriad contexts, has only conferred respectability on a phrase whose intended meaning often does not amount to much more than: "Do me a favour" or: "Are they having a laugh?" In Extraordinary Popular Delusions & the Madness of Crowds, published in 1841, Charles Mackay described the rapidity with which vacuous or ridiculous phrases could take hold. Suddenly, he recorded, everyone in London would be shouting: "Quoz!" or: "What a shocking bad hat!" or: "Has your mother sold her mangle?" The last one did not, apparently, last long. "What tended to impede its progress," Mackay explained, "was that it could not be well applied to the older portions of society."

When a phrase really comes into favour, he wrote: "It seems applicable to every circumstance and is the universal answer to every question." But even so, "Quoz!" was never the stuff of political discourse. A couple of weeks ago, the scarcely more elegant "He just doesn't get it" arrived in the House of Commons, where it was endorsed, not unfittingly, by talented local teenager Nick Clegg.

"Does he not understand the extreme danger to our democracy when people start feeling that there is simply no one in charge?" Clegg demanded. He concluded, with what must have seemed to him a scatter of Socratic stardust: "The Prime Minister just doesn't get it!" Perhaps he was right, for this now-threadbare thrust was widely reported.

Evidently adolescent sulkers are cleverer than we thought. "You just don't get it" turns to be much more than a simple, limitlessly versatile insult, ideal for screaming in the seconds before the door slams behind you. Deployed either before or after a brief statement of the obvious, eg: "We're sick and tired of politicians/bankers/television executives treating the public like idiots", it asserts enhanced understanding on the part of the speaker at the same moment that it refuses, on principle, to supply any evidence to support this claim. Why bother? What's the point of sharing your ideas for a post-capitalist economy or for a truly representative system of parliamentary democracy with individuals who are too hardened or corrupt to get it anyway? What is "it"? If you have to ask, you'll never know.

How did people manage before TJDGI? Perhaps they had less to provoke them. In defence of "They just don't get it" users, maybe a vacancy arose, in the early summer of 2009, for a phrase that aptly expresses the helpless, incredulous disgust provoked by an apparently inexhaustible succession of public failures and scandals, arriving in a continually deepening recession.

Even when they're insulted beyond words, people have to find something to say. Or howl, before they construct a considered response to a week like the last one, in which, against the usual background of de-redacted expenses revelations, Blair nobbled the Iraq inquiry; Labour made Bercow Speaker; the boss of a publicly owned bank got a £1.2m starting salary; the Tories revealed the joys of shadow cabinet moonlighting; City traders exulted over the return of bonuses; and even the BBC started wheedling that its executives deserved to be specially rewarded.

If not, the Corporation reminded us, a person of Jana Bennett's stature might be forced to work for one of those mythical private companies where the handbags and haircuts arrive gratis, like little people's free salt and ketchup.

If anything can end the chorus of "They just don't get it", it will surely be the inadequacy of even that hard-working phrase to cover the millions of ways in which so many people don't get it, since it's obvious that different offenders must fail to different degrees. Who, out of Brown and Cameron, gets it even less than the other? Which profession tops the they-just-don't-get-it league? For all the BBC's self-congratulatory gratuities and the abject fiddling of Blears, Hoon and their fellow speculators, the scale of the City's indifference, as a publicly supported parasite, should surely put it near the top.

But should the City come before or after a governing party that is still so unwilling to respond to public fury and clean up banks it controls? After months of public fulmination, one thing is clear: if telling people they just don't get it changed anything, it would have happened by now. For repeat offenders, it's time to think of something new.